A mirror plane as a regulating agent of crystal structures of sulfides. Rare earth sesquisulfides

By means of separating dose-packed atomic surfaces in three most widely spread structural types of rare earth sesquisulfides - monoclinic Ho2S3 and rhombic Gd2S3 and Tm2S3 - it has been shown that frequent occurrence in sulfide structures of mirror symmetry planes separated by a distance of about 2A is accompanied by the presence of trigon nets of both cations and anions filled by the splitting principle. The mirror symmetry plane actually deprives them of one degree of freedom out of three and stabilizes monolayer hexagonal package